The present invention generally relates to transducers that are used to help monitor a fluid flowing through, for example, a canal, conduit, duct, pipe, tube, or the like. The present invention more particularly relates to ultrasonic transducers that are used to help monitor, in a non-intrusive manner, the flow rate or temperature of a fluid flowing through a duct.
To propel a rocket into the upper atmosphere and outer space, a propellant is provided to a thrusting engine situated onboard the rocket. In most liquid-propellant type rocket engines, the propellant primarily includes a mixture of both fuel and an oxidizer. Typically, the fuel is gasoline, kerosene, alcohol, or liquid hydrogen. The oxidizer, on the other hand, is typically a cold liquefied gas such as liquid oxygen. In general, the fuel and oxidizer are individually pumped from separate tanks, conveyed along separate pipes or ducts, and ultimately delivered into a common combustion chamber associated with the thrusting engine. Once within the combustion chamber, the fuel and oxidizer together serve as a propellant mixture which is ignited and burned, thereby creating both a high-pressure and high-velocity stream of hot gases. This stream of hot gases is directed through a nozzle wherein the stream is further accelerated until the gases leave the engine area and are ultimately expelled from the thrusting end of the rocket. By expelling the stream of gases in this manner, the rocket itself is thereby thrust into the upper atmosphere and outer space in a direction generally opposite its thrusting end.
To attain a proper level of thrust, the individual amounts of fuel and oxidizer delivered to the combustion chamber of a thrusting engine are closely regulated and monitored. In doing so, an optimum fuel-to-oxidizer propellant mixture ratio for a desired level of thrust is thereby achieved. In the case wherein the oxidizer is liquid oxygen, the actual flow rate of the liquid oxygen passing through its respective duct is both sensed and monitored to thereby provide feedback control on the amount of liquid oxygen that is actively being pumped and ultimately delivered to the combustion chamber. In this way, efficient propellant consumption by a thrusting engine is realized via real time control. As a result, inadvertent delivery of the rocket and its payload into a lower-than-anticipated orbit, for example, is largely prevented.
Traditionally, in-flow turbine flowmeters have been utilized for helping monitor the flow rate of liquid oxygen being delivered into a combustion chamber. In general, such in-flow turbine flowmeters are situated within the liquid oxygen delivery duct itself wherein the flowmeters physically encounter and interact with the flow of liquid oxygen passing therethrough. By interacting with the flow of liquid oxygen in this manner, the turbine flowmeters are thereby able to generate electrical feedback control signals that are representative of the real time flow velocity of the liquid oxygen passing through the duct. Although such in-flow turbine flowmeters are generally effective in helping monitor the flow rate of liquid oxygen, there are certain disadvantages in utilizing such flowmeters. First, given that eddies and turbulence within a moving fluid tend to skew overall flow velocity, installing one or more of such in-flow turbine flowmeters within a duct pursuant to a positioning scheme that attempts to accurately detect fluid flow velocity is oftentimes challenging and difficult. Second, if an in-flow turbine flowmeter experiences structural problems or damage while situated in a duct, such can sometimes render a thrusting engine altogether unusable. Third, such in-flow turbine flowmeters are relatively bulky both in terms of size and mass, thereby undesirably reducing the payload carrying capacity of a rocket. Fourth, the flow-intrusive or flow-invasive nature of such in-flow turbine flowmeters sometimes gives rise to a small parasitic pressure drop within a duct that can somewhat reduce the operating efficiency of a thrusting engine.
In recent years, ultrasonic transducer flowmeters have gained popularity due to their non-intrusive manner of operation. In a most common and basic form, an ultrasonic transducer flowmeter generally includes two lightweight piezoelectric ceramic wafers that are both mounted on and coupled to the outer surface of a duct. The two wafers are generally mounted on opposite sides of the duct such that they are slightly offset from each other along the length of the duct. In this way, one wafer is situated upstream and the other wafer is situated downstream along the length of the duct. Mounted as such, the two wafers are also electrically connected via separate wires to a common electric control circuit that is capable of transmitting and receiving electrical signals to and from the two wafers. In such a configuration, the control circuit initially transmits, for example, an electrical signal along a wire to the upstream wafer. Upon receiving the electrical signal, the upstream wafer immediately converts the electrical signal into an ultrasonic acoustic signal and directionally transmits the acoustic signal through the duct and toward the downstream wafer. Upon receiving the acoustic signal, the downstream wafer immediately converts the acoustic signal into an electrical signal which is then conducted along a wire and received by the control circuit. Upon receiving the electrical signal, the control circuit then calculates the downstream transit time of the acoustic signal through the duct based on the elapsed time between the initial transmission of an electrical signal to the upstream wafer and the later receipt of an electrical signal from the downstream wafer. Thereafter, the control circuit then transmits an electrical signal along a wire to the downstream wafer. Upon receiving the electrical signal, the downstream wafer immediately converts the electrical signal into an ultrasonic acoustic signal and directionally transmits the acoustic signal through the duct and toward the upstream wafer. Upon receiving the acoustic signal, the upstream wafer immediately converts the acoustic signal into an electrical signal which is then conducted along a wire and received by the control circuit. Upon receiving the electrical signal, the control circuit then calculates the upstream transit time of the acoustic signal through the duct based on the elapsed time between the initial transmission of an electrical signal to the downstream wafer and the later receipt of an electrical signal from the upstream wafer. After both the downstream and upstream transit times have been calculated in this manner, the control circuit then compares the two transit times and calculates their difference. Given that an acoustic signal's transit time through a given fluid is generally directly affected by the fluid's flow velocity, the transit time difference, once calculated, is then used by the control circuit to substantially determine the real time velocity of the fluid passing through the duct. After determining the fluid velocity, known dimensions of the duct such as its inner diameter and associated cross-sectional area can then be utilized to determine a fluid flow rate by multiplying the velocity of the fluid by the cross-sectional area of the duct. Once the flow rate is determined, the volume and amount of fluid actually being delivered via the duct during a period of time can then generally be determined and thereafter adjusted as necessary.
In mounting the two piezoelectric ceramic wafers of an ultrasonic transducer flowmeter on a duct, the wafers must both be intimately attached or coupled to the duct so that there generally is no air gap or separation between the wafers and the outer surface of the duct. The reason for such is because any air gap, separation, or delamination between a wafer and the outer surface of the duct tends to reflect and unduly interfere with any ultrasonic acoustic signals being directed and transmitted between the two wafers. Hence, if separation between even one of the two wafers and the outer surface of a duct is significant enough, proper transmission and receipt of acoustic signals between the two wafers will no longer be possible. Consequently, the electric control circuit will not be able to accurately determine the flow rate of the fluid passing through the duct.
When fluid flow through a duct occurs in an operating environment wherein noncryogenic temperatures are involved, certain gels, epoxies, and greases can effectively be utilized to help intimately mount and couple the piezoelectric ceramic wafers of an ultrasonic transducer flowmeter to the outer surface of the duct. In, however, an operating environment onboard a liquid-propellant type rocket wherein cryogenic liquids are commonly circulated about the super-hot combustion chamber and nozzle for cooling and other purposes, wafer-coupling media such as gels, epoxies, and greases are generally not very effective. In particular, if a gel, epoxy, or grease medium is used, for example, to couple a wafer to a duct in which liquid oxygen in its cold liquefied form is being conveyed, the medium will typically freeze since liquid oxygen temperatures commonly extend below 300° F. Such freezing in conjunction with the extreme and violent vibration caused by an operating thrusting engine typically causes micro-cracking, fracture, and delamination in the medium. Consequently, the wafer is thereby separated, to some degree, from the outer surface of the duct, thereby rendering the liquid oxygen flow rate potentially undeterminable.
Due to the above-intimated heretofore lack in flowmeters that are characteristically both non-intrusive and robust in nature, the flow rate of liquid oxygen passing through a duct onboard a liquid-propellant type rocket is generally not directly determined according to current practice. Instead, engineers have alternatively had to resort to more indirect and less accurate methods in determining the flow rate of liquid oxygen onboard a liquid-propellant type rocket. Having to settle for such indirect methods, however, is generally undesirable, for such methods tend to facilitate less than optimum thrusting engine performance and also briefly mask the need for thrusting engine maintenance. With regard to thrusting engine performance, direct and accurate real time determinations of liquid oxygen flow rates are highly preferred for feedback control purposes so that timely delivery of propellant constituents into the combustion chamber can be precisely regulated. In this way, propellant mixture ratios within the combustion chamber can be timely tweaked to thereby facilitate efficient propellant consumption and optimum thrusting engine performance. With regard to thrusting engine maintenance, direct and accurate real time determinations of liquid oxygen flow rates are also highly preferred for making close comparisons to predicted flow rate values derived from a predetermined ideal engine operation model. In this way, any significant deviations of such accurately determined real time flow rates from predicted engine model flow rates may be considered more seriously. As a result, one or more various causes for anomalous engine operation can be detected, isolated, and addressed in an early and timely fashion. Such early detection is highly desirable, for such helps reduce repair and maintenance costs when progressive type problems arise. In addition, such early detection is also highly desirable since such helps improve overall safety.
In light of the above, there is generally a present need in the art for flowmeters that are characteristically both non-intrusive with respect to fluid flow and robust in various extreme operating environments. More particularly, there is a present need in the art for ultrasonic transducer flowmeters that are largely able to both endure cryogenic temperatures and withstand extreme vibration onboard a rocket without experiencing delamination from a duct conveying, for example, liquid oxygen.